The present invention relates to pyroelectric detectors, particularly but not exclusively in their application to detecting and classifying moving bodies.
Single element pyroelectric detectors may be used in large quantities in applications such as intruder alarms, fire alarms and monitoring the movement of vehicles.
A simple pyroelectric infra-red detector is formed from a slice of pyroelectric material with metal electrodes on opposite faces and with its polar axis perpendicular to the electrodes. At constant temperature, the spontaneous polarization is compensated for by surface charges which accumulate via leakage paths between the two faces. If the temperature of the pyroelectric material is changed the spontaneous polarization changes in proportion to the pyroelectric co-efficient. The change in polarization appears as a charge on the electrodes.
A common requirement for infra-red detectors is to respond to the movement of a body, for example the movement of a person or vehicle, perhaps many metres away. The incident radiation from such a source may produce a change in temperature at the detector as low as 10.sup.-6 .degree. C. resulting in the generation of a charge of perhaps as low as 10.sup.-16 coulombs on a capacitance of 10 pF. In order to detect these very small changes, low noise high impedance amplifiers must be employed, such as for example a JFET source follower.
If a detector is used in an environment where the ambient temperature is fluctuating or the background temperature which the detector is viewing is not stable, then large unwanted `noise` signals will be produced. In order to overcome this problem, compensated detector configurations have been developed.
Compensation may be obtained for example by connecting two oppositely polarized detector elements either in series or in parallel (the two elements should be of equal area if connected in parallel) but having one element absorbing and another element reflecting to the infra-red radiation to be detected. Since environmental changes affect both elements alike, the signals from the two elements cancel each other and no output is observed. Greater efficiency may be obtained where a moving target is viewed by arranging the system so that the image moves from one element to the other.
In a known monolithic compensated pyroelectric detector element structure different spaced metallised areas used as electrodes may be made absorbing or reflecting according to the particular application required. For many applications a null output is required for radiation falling on the unmetallised areas of the pyroelectric material; this is unlikely to be achieved owing to partial heat absorption in the pyroelectric material which will be sensed by the detectors. The deposition of an isolated reflecting metallisation of, for example, gold or aluminium in the space between the electrodes may sometimes be undesirable since an unwanted capacitance to ground is thereby generated so degrading the detector performance. The present invention seeks to overcome these problems.